A Strategy on Floods: Don’t Fight the Water
Cities around the world face a daunting challenge in the era of climate change: Supercharged rainstorms are turning streets into rivers, flooding subway systems and inundating neighborhoods, often with deadly consequences.
Kongjian Yu, a landscape architect and professor at Peking University, is developing what might seem like a counterintuitive response: Let the water in.
“You cannot fight water,” he said. “You have to adapt to it.”
Instead of more drainage pipes or flood walls, Mr. Yu wants to dissipate the destructive force of floodwaters by slowing them and giving them room to spread out.
He calls the concept “sponge city” and says it is like “doing tai chi with water,” a reference to the Chinese martial art in which an opponent’s energy and moves are redirected, not resisted.
Through his Beijing-based company, Turenscape, one of the world’s largest landscape architecture firms, Mr. Yu has overseen the development of hundreds of landscaped urban water parks in China where runoff from flash floods is diverted to soak into the ground or be absorbed into constructed wetlands.
Currently, 65 percent of urban areas in China experience some degree of flooding each year, according to Mr. Yu. He noted that much of China has a monsoon climate subject to extremely heavy bursts of rain that pose an increasing hazard as climate change advances — warm air can hold more moisture, resulting in heavier rainstorms.
The sponge city program was inaugurated by President Xi Jinping in 2015 with pilot projects in 16 cities and has expanded to more than 640 sites in 250 municipalities around the country.
You can see the concept in Houtan Park, a nearly kilometer-long strip of greenery along the Huangpu River in Shanghai that Mr. Yu designed on a former industrial site. Terraces planted with bamboo and native forbs and grasses are bisected by wooden walkways that zigzag between ponds and constructed wetlands. The wetlands filter water, slow the river’s flow and provide habitat for waterfowl and spawning fish.
The goal is that by 2030, 70 percent of the rain that falls on China’s sponge cities during extreme weather events should be absorbed locally, not accumulate in the streets.
Whether enough land can be converted is a key question. Edmund Penning-Rowsell, a research associate at the University of Oxford who focuses on water security, said the scale of the sponge city projects would have to be huge to cope with flooding on their own.
Where large tracts of land are not available, sponge city projects are replacing concrete and asphalt with permeable pavement, installing green roofs and creating trenches called bioswales that channel storm-water runoff and use vegetation to filter out debris and pollution.
The sponge city concept is not unique to China. One of Mr. Yu’s projects is the Benjakitti Forest Park in Bangkok, a maze of ponds, trees and miniature islands that opened to the public in 2022 and occupies over 40 hectares on the site of a former tobacco factory.
Separately, in 2007 the Dutch government began a program, Room for the River, that consists of more than 30 projects around four rivers, including the Rhine. Philadelphia, Pennsylvania, and Malmo, Sweden, also have projects.
Zhengzhou, in northeastern China on the Yellow River, was an early adopter of the sponge city concept, spending hundreds of millions of dollars building projects from 2016 to 2021. But torrential rains inundated much of the city in July 2021, creating scenes of destruction and killing hundreds, including at least 14 in a subway tunnel.
Why were the floods so disastrous? Mr. Yu said some of the money for sponge projects was diverted to other programs and that the land set aside for them was insufficient.
If permeable surfaces or green spaces make up 20 percent to 40 percent of a city’s area, he said, “you can virtually solve the problem of urban inundation.”